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101 lines
3.7 KiB
101 lines
3.7 KiB
/*
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* Asterisk -- An open source telephony toolkit.
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*
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* Copyright (C) 2007, Digium, Inc.
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*
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* Tilghman Lesher <tlesher AT digium DOT com>
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*
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* See http://www.asterisk.org for more information about
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* the Asterisk project. Please do not directly contact
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* any of the maintainers of this project for assistance;
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* the project provides a web site, mailing lists and IRC
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* channels for your use.
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*
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* This program is free software, distributed under the terms of
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* the GNU General Public License Version 2. See the LICENSE file
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* at the top of the source tree.
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*/
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <sys/time.h>
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#include <sys/resource.h>
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#include <utime.h>
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#include <fcntl.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <string.h>
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/*!\brief
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* At one time, canaries were carried along with coal miners down
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* into a mine. Their purpose was to alert the miners when they
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* had drilled into a pocket of methane gas or another noxious
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* substance. The canary, being the most sensitive animal would
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* immediately fall over. Seeing this, the miners could take
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* action to escape the mine, seeing an imminent danger.
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*
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* This process serves a similar purpose, though with the realtime
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* priority being the reason. When a thread starts running away
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* with the processor, it is typically difficult to tell what
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* thread caused the problem, as the machine acts as if it is
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* locked up (in fact, what has happened is that Asterisk runs at
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* a higher priority than even the login shell, so the runaway
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* thread hogs all available CPU time.
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*
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* If that happens, this canary process will cease to get any
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* process time, which we can monitor with a realtime thread in
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* Asterisk. Should that happen, that monitoring thread may take
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* immediate action to slow down Asterisk to regular priority,
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* thus allowing an administrator to login to the system and
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* restart Asterisk or perhaps take another course of action
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* (such as retrieving a backtrace to let the developers know
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* what precisely went wrong).
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*
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* Note that according to POSIX.1, all threads inside a single
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* process must share the same priority, so when the monitoring
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* thread deprioritizes itself, it deprioritizes all threads at
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* the same time. This is also why this canary must exist as a
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* completely separate process and not simply as a thread within
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* Asterisk itself.
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*/
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const char explanation[] =
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"This file is created when Asterisk is run with a realtime priority (-p). It\n"
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"must continue to exist, and the astcanary process must be allowed to continue\n"
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"running, or else the Asterisk process will, within a short period of time,\n"
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"slow itself down to regular priority.\n\n"
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"The technical explanation for this file is to provide an assurance to Asterisk\n"
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"that there are no threads that have gone into runaway mode, thus hogging the\n"
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"CPU, and making the Asterisk machine seem to be unresponsive. When that\n"
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"happens, the astcanary process will be unable to update the timestamp on this\n"
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"file, and Asterisk will notice within 120 seconds and react. Slowing the\n"
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"Asterisk process down to regular priority will permit an administrator to\n"
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"intervene, thus avoiding a need to reboot the entire machine.\n";
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int main(int argc, char *argv[])
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{
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int fd;
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/* Run at normal priority */
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setpriority(PRIO_PROCESS, 0, 0);
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for (;;) {
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/* Update the modification times (checked from Asterisk) */
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if (utime(argv[1], NULL)) {
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/* Recreate the file if it doesn't exist */
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if ((fd = open(argv[1], O_RDWR | O_TRUNC | O_CREAT, 0777)) > -1) {
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write(fd, explanation, strlen(explanation));
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close(fd);
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} else {
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exit(1);
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}
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continue;
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}
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/* Run occasionally */
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sleep(5);
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}
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/* Never reached */
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return 0;
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}
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